My 2005 model's dome light functions intermittently.

[Sternwheeler]

Here's what I know: The light, when it does not work will not respond to the roller switch in the dash or to either open door. When it does work, it works for all. No other electrical function in the vehicle fails to operate, including 4x4 activation. This is a 4.0 XLT without power windows or locks.

What I think I know: This seems to be a failure in the Battery Saver Relay. This relay is part of the Smart Junction Box. Said SJB is masquerading as a fuse panel behind the passenger kick-panel. Please correct anything I've screwed up here.

Is the relay replaceable? Every time I believe I've gained some understanding about this item, it turns out I'm looking at the wrong year-model. I've seen front-and-rear pictures of the SJB and saw nothing I recognize, but perhaps it's internal.

I'm getting ready to add aftermarket cruise control and a new stereo, and I don't want to introduce new electrical components into a system that is not 100%.

Thank you for help!

 

[Dirtman]

May wanna back up a bit before digging into the SJB.... have you simply opened up the light and checked the ground and connections for corrosion?

 

[rusty ol ranger]

My expedition did this at one point and it was one of the switches that tells the lights the doors open.

Does the key in chime work?

 

[Grumpaw]

My expedition did this at one point and it was one of the switches that tells the lights the doors open.

Does the key in chime work?

X2...
There's a door open switch, which also operates the dome light, attaches to each of the door lock, inside the door. Have to remove the door panel
to access. Common problem when switches get wet/dirty. Switch can stick causing it to work/not work, or work intermittently
If your door ajar dash light is also on or flashes intermittently when door is closed, than its most likely the switch.
And, as Dirtman posted, check the grounds at the dome light itself. If bulb is good, intermittent operation is often a bad ground..
Grumpaw

 

[Josh B]

In that trio, I'd look first at the dome light itself. If it doesn't work on both doors, and it does work on both doors, would lead me to believe it's somewhere other than the doors

 

[Uncle Gump]

That is where every electrical diagnosis should begin with a solid look at the wiring diagrams. Fully understand the symptom(s)... apply it to the diagrams... look for a common point of failure. We called it driver seat diagnostics...

 

[Sternwheeler]

May wanna back up a bit before digging into the SJB.... have you simply opened up the light and checked the ground and connections for corrosion?

Yessir. Sorry it took so long to reply, but I've had some work issues that interfered. I've actually had the light cut itself so long after I activated the switch that I forgot the switch was on.

 

[Sternwheeler]

My expedition did this at one point and it was one of the switches that tells the lights the doors open.

Does the key in chime work?

Yes, but I've never thought to check when the light was inop. I'll do so the next time it fails.

 

[Sternwheeler]

X2...
There's a door open switch, which also operates the dome light, attaches to each of the door lock, inside the door. Have to remove the door panel
to access. Common problem when switches get wet/dirty. Switch can stick causing it to work/not work, or work intermittently
If your door ajar dash light is also on or flashes intermittently when door is closed, than its most likely the switch.
And, as Dirtman posted, check the grounds at the dome light itself. If bulb is good, intermittent operation is often a bad ground..
Grumpaw

I've had normal operation from the door-ajar light. Could the door switches interfere with the dash switch?

 

[Sternwheeler]

In that trio, I'd look first at the dome light itself. If it doesn't work on both doors, and it does work on both doors, would lead me to believe it's somewhere other than the doors

Right, and as the dash switch will not turn on the light either, and as the ground at the lamp is good, I was looking elsewhere. I figured there was no way that both door switches and the dash switch could all fail and recover at the same time every time.

 

[Sternwheeler]

That is where every electrical diagnosis should begin with a solid look at the wiring diagrams. Fully understand the symptom(s)... apply it to the diagrams... look for a common point of failure. We called it driver seat diagnostics...

I was trying, but some of the circuit is controlled by a "Low Current Board" that is not diagrammed. It looks like it controls flow of power to the door switches and the door-ajar lights along with the grounds to operate the relay. It also seems to receive a signal from the dash switch to trigger the left side of the relay, I guess. Electrical is not my strong suit.